Amine curing epoxy formulations as curable compositions are widely applied in the formulation of adhesives, sealants, and coatings. The curing reaction, which comprises the addition of amines and epoxides, suffers from its volume shrinkage up to levels as high as 7% shrinkage. The volume shrinkage during the curing reaction causes the formation of voids and micro-cracks in the cured material, and thus results in lowering the mechanical strength of the material. Especially in the application of adhesives, shrinkage causes contact failure between the adhesive and the substrate, thus leading to adhesion failure. In addition, moisture can reach the surface of the substrate through voids and cracks, causing corrosion of the surface.
As a method to reduce shrinkage during the curing reaction between epoxides and amines, the addition of cyclic (di)thiocarbonate is effective. However, the shrinkage values are still not satisfactory for high precision adhesive applications and thus must be improved. Furthermore, due to the high reactivity between cyclic dithiocarbonates (DTC) and amines, DTC immediately reacts with amine in the formulation, even at ambient temperature to cause the disadvantages of a short pot life of the formulation and when the curing reaction is too rapid, an increase of viscosity of the formulation before the amine-hardener is completely homogenized into the formulation.
Formulations containing epoxides, dithiocarbonates, and amino compounds have recently been examined by Sanyo Kasei Co. Ltd. in the Japanese Laid-Open Patent Applications Nos. JP2003-41227, JP2002-317047, JP2002-302530, and JP2001-288242.
JP2003-41227 describes a contact adhesive. Two surfaces, the first one being coated with a mixture of epoxides and DTC, the second one being coated with amines, can be adhered upon their contact. By separating DTC and amine, the reaction does not start until the two surfaces are contacted. However, the adhesion performance is affected by ambient conditions, because the curing reaction depends on the diffusion of the components of the curing composition.
JP2002-317047 describes the utilization of ketimines, which release the corresponding amine upon hydrolysis by moisture. With this technology, it is possible to control the reaction rate of DTC with amine. However, curable epoxy formulations using ketimines have a disadvantage when being used for shrinkage-sensitive applications. The regeneration of the corresponding amine from the ketimine generates volatile ketones, which will be released from the formulation upon curing reaction to cause volume shrinkage. Even if non-volatile ketones are formed, they will remain in the cured resin to cause a decrease in the resin's mechanical and chemical properties. This patent application further describes, that if needed, unsaturated carboxylic acids can be added for trapping the thiol group which is generated by the reaction of DTC with amines. However, the definition of “unsaturated carboxylic acid” in the description of the invention clarifies that a compound obtained by esterification of aliphatic alcohol or phenol with (meth)acrylic acid is meant and not the carboxylic acid itself.
JP2002-302530 claims a curable formulation composed of cyclic thio compounds, cyclic ethers, and a molecule having more than one nucleophilic group. The preferred nucleophilic group is the amino group. Carboxylate groups are described as candidate nucleophilic groups in the description of the patent application, however, no combination with amino compounds is described.
JP-2001-288242 discloses a curable and foamable formulation composed of epoxides, cyclic dithiocarbonates, acid anhydrides, and an amine-type catalyst. It is claimed that the composition shows volume expansion upon curing reaction. However, volume expansion is due to the formation of gases such as CS2 and COS, caused by decomposition of dithiocarbonate. It is apparent that the formation of these harmful gases limits the application of the formulation. Furthermore, it also causes decrease in the mechanical strength of the cured material.
Therefore it was one major aim of the present invention to overcome the above drawback and to provide amine curing epoxy formulations with an improved adhesion performance compared to those known in the prior art. In particular formulations with low volume shrinkage upon curing, which are suitable in high precision applications, such as high precision adhesive bonding, should be provided. Such compositions should further show an enhanced pot life ensuring a high performance in processability. In addition they should lack foam formation during the curing reaction due to formation of harmful gases, thus providing rigid cured materials.